Biodegradable corrosion inhibitors of low toxicity

ABSTRACT

A method for inhibiting corrosion in an aqueous medium is disclosed. The method comprises incorporating into the medium an amount of a water-soluble corrosion inhibitor sufficient to inhibit corrosion. The corrosion inhibitor comprises an N-ethoxy,2-substituted imidazoline, the N-ethoxy substituent having from one to about thirty ethoxy units and the 2-substituent being an unsaturated or polyunsaturated fatty chain of from about six to about thirty carbons. If the medium is sweet, the inhibitor further comprises a phosphate ester derived from a water-soluble oxyethylated, straight chain alcohol of from about two to about thirty carbons. Corrosion inhibitor compositions useful in such method are also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to corrosion inhibition and moreparticularly to inhibition of corrosion in environmentally sensitiveaqueous media.

2. Description of Prior Art

Corrosion of metal surfaces in aqueous media has long been anintractable problem. This problem is especially troublesome in deep seaoperations such as off-shore oil production, where corrosion inhibitorsmust satisfy several criteria in order to be effective in the demandingconditions encountered in such operations. A number of corrosioninhibitors have been developed in attempts to satisfy the demandsimposed by such activities. However, because it is difficult to meeteach of several independent corrosion inhibition criteria, these effortshave met with varying, but not totally satisfactory, success.

Nevertheless, increasing environmental concerns have introduced evenfurther criteria for corrosion inhibitors to satisfy. In particular, thecorrosion inhibitor should be compatible with the sensitive life formsindigenous to the medium into which the inhibitor is incorporated.

For example, in North Sea operations, survival not only of fish, butalso, for instance, of algae and the microorganism Skeletonema costatumis of concern. Thus, environmental constraints have been imposed on thetypes of compositions used in the North Sea, thereby to protect suchorganisms. However, commonly available commercial inhibitors have beenfound to be too toxic to the organism. More specifically, even aconcentration of less than one part per million by weight (ppm) ofconventional inhibitors has been found to retard growth of Skeletonemacostatum test populations by 50% in 96 hours. This may be written asEC₅₀ <1 ppm. Thus, a corrosion inhibitor having an EC₅₀ greater than 1ppm is desired.

In addition, it is desired that the inhibitor meet several otherenvironmental criteria as well. For example, the inhibitor should besufficiently biodegradable so that within 28 days after treatment, theinhibitor has degraded at least 70% in terms of the theoretical oxygenconsumption required for complete degradation (i.e.; the biochemicaloxygen demand BOD-28≧70%). Of course, it is more desirable that completedegradation occur within 28 days. That is, the goal is BOD-28=100%.

Further, the water-solubility of the inhibitor should be sufficient toavoid or to minimize bio-accumulation that otherwise can result with fatsoluble inhibitors in lower life forms. The fat soluble inhibitors maybecome more concentrated as they move up the food chain. This may bequantified by measuring the resulting concentration of inhibitor in theoctanol phase and in the water phase of an n-octanol/water medium intowhich the inhibitor has been injected, and dividing the former by thelatter. It is desired that the logarithm (base 10) of that quotient beless than three. Stated another way, log K or "partitioning" should beless than three in terms of a log value.

Moreover, because evaporation of a toxic solvent (if any) would beundesirable, the Solvent evaporation factor (YL) should not be greaterthan three. And, because of the dangers of flammability, the flash pointshould be greater than 56° C.

Another problem that has been encountered is the tendency of someinhibitors to form a floc with calcium ions that may be present in themedium to be treated, clouding the medium and short-circuiting theeffectiveness of the inhibitor. Accordingly, the inhibitor should nottend to form a floc with calcium ions as well as meet the significantefficacy and environmental criteria.

Commercial inhibitors generally have not been found to meet suchdemanding criteria, especially while still affording highly effectivecorrosion inhibition. Thus, inhibitors that not only providesatisfactory corrosion inhibition, but satisfy such environmentalconcerns as well, are still being sought. In fact, because of increasingenvironmental emphasis, the search is intensifying.

SUMMARY OF THE INVENTION

The present invention, therefore, is directed to a novel method forinhibiting corrosion in an aqueous medium. The method comprisesincorporating into the medium an amount of a water-soluble corrosioninhibitor sufficient to inhibit corrosion. The corrosion inhibitorcomprises an N-ethoxy,2-substituted imidazoline. The N-ethoxysubstituent has from one to about thirty ethoxy units and the2-substituent is an unsaturated or polyunsaturated fatty chain of fromabout six to about thirty carbons. If the medium is sweet, the inhibitorfurther comprises a phosphate ester derived from a water-solubleoxyethylated derivative of a straight chain alcohol of from about two toabout thirty carbons.

The present invention is also directed to a corrosion inhibitorcontaining such imidazoline and phosphate ester in an environmentallyacceptable solvent such as water or a glycol or a mixture thereof.

Among the several advantages of this invention, may be noted theprovision of a method for inhibiting corrosion in environmentallysensitive aqueous media; the provision of such method that isenvironmentally compatible with such media; the provision of such methodthat does not tend to form a floc with calcium ions that may be in themedia; and the provision of a corrosion inhibitor that may be used insuch method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, it has been discovered thatincorporating into a medium a corrosion inhibitor that comprises anN-ethoxy,2-substituted imidazoline, the N-ethoxy substituent having fromone to about thirty ethoxy units and the 2-substituent being anunsaturated or polyunsaturated fatty chain of from about six to aboutthirty carbons and, if the medium is sweet, that further comprises aphosphate ester derived from a water-soluble oxyethylated derivative ofa straight chain alcohol of from about two to about thirty carbons,affords surprisingly superior corrosion inhibition with little or nodeleterious environmental effects on the medium. Moreover, no tendencyto form a floc with calcium has been observed.

Compositions having little or no deleterious environmental effects on amedium of concern are described herein as "environmentally compatible".This requirement should include such considerations as toxicity,water-solubility, biodegradability, and so forth. Non-toxicity isreferred to herein. However, it is recognized that nearly any substancemay be toxic at some concentration. As used herein, therefore,non-toxicity refers to very low toxicity in the concentrations to beemployed. For example, for off-shore drilling and production, it refersto compositions having an EC₅₀ greater than 1 ppm by weight forSkeletonema costatum. With respect to biodegradability, a BOD-28 ofgreater than 70% is desired.

The N-ethoxy,2-substituted imidazoline may be prepared by reacting anN-substituted ethylene diamine with a fatty acid to form an imidazolineand ethoxylating the imidazoline if necessary. The N-substitutedethylene diamine is of the form H₂ NCH₂ CH₂ NHRMH, wherein R is anorganic moiety and --MH represents a terminal group that includes ahetero atom such as oxygen, nitrogen or sulfur and at least onehydrogen, thereby to provide a site for attachment of ethylene oxide.The organic moiety may be any of a wide range of possibilities, but amoiety that is environmentally compatible should be selected. Althoughthe organic moiety may include nitrogen atoms that would render theN-substituted ethylene diamine a polyamine of three or more aminogroups, for the sake of simplicity and in view of the fact that the twonitrogen atoms identified explicitly in the formula above are thoserelevant to the functionality of concern, the reactant will be referredto herein simply as an N-substituted ethylene diamine. Nevertheless, itshould be understood that higher polyamines are contemplated if thepresence of further amino groups beyond the noted two is a result ofinclusion in the R group. Preferably, R is an alkylene, an arylene, oran aralkylene. More preferably, R is ethylene, isopropylene or --(CH₂CH₂ O)_(n) (CH₂ CH₂)-- wherein n is an integer from 1 to about 30. Evenmore preferably, R is ethylene or the group --(CH₂ CH₂ O)_(n) (CH₂CH₂)-- wherein n is an integer from about one to about seventeen. Mostpreferably, R is ethylene.

The group --MH, as noted, should provide a site for attachment ofethylene oxide for ether or polyether formation. Preferably, --MHrepresents --OH, --NH₂ or --SH, most preferably --OH. Thus, preferredN-substituted ethylene diamines include, for example, NH₂ CH₂ CH₂NH--CH₂ CH₂ OH, NH₂ CH₂ CH₂ NH--CH₂ CH(CH₃)OH and NH₂ CH₂ CH₂ NH--CH₂CH₂ NH₂, most preferably, NH₂ CH₂ CH₂ NH--CH₂ CH₂ OH.

The fatty acid also should be selected for environmental compatibility.Thus, a preferred fatty acid is a mono- or poly-unsaturated fatty acidof from six to about thirty, preferably about twelve to about twentycarbons. The term polyunsaturated refers to two or more points ofunsaturation. Thus, the fatty acid is of the form R'COOH, wherein R' isan unsaturated or polyunsaturated chain of from about five to aboutthirty carbons, preferably about eleven to about twenty carbons.Particular suitable fatty acids have been found to be oleic, linoleicand eladeic acid.

The diamine and fatty acid are reacted in about a 1:1 molar ratio undera vacuum with the addition of heat, such as up to about 240° C., untilall water is removed. The resulting imidazoline is then ethoxylated ifnecessary to build the N-substituent of the imidazoline to include atotal of from one to about thirty ethoxy units as necessary to renderthe product water-soluble. As used herein, the term water-soluble meansmiscible with water at the concentration to be employed for corrosioninhibition. Thus, the resulting product is of the form ##STR1## whereinB and B' (the residue of the fatty acid) are as defined above, M is theresidue from the --MH group after removal of the B, preferably --O--,--NH-- or --S--, most preferably --O--, x is 0 or 1 and y is an integerfrom zero to about thirty selected such that the total number of ethoxyunits in the N-substituent is from one to about thirty (preferably twoto about eighteen), depending on the number necessary to render theimidazoline water-soluble.

The phosphate ester is such as those described in, for example, U.S.Pat. No. 4,339,349 to Martin (a present co-inventor) et al. or U.S. Pat.No. 3,846,071 to Redmore. In particular, the phosphate ester may beprepared by reacting an ethoxylated alcohol with polyphosphoric acid orwith phosphoric anhydride. Thus, the first step may involve ethoxylatingan alcohol. Generally, the alcohol is environmentally compatible and sois one that is non-toxic to aquatic life in the medium to be treated andbiodegradable and can be made water-soluble upon ethoxylation.Typically, therefore, a C₂ to C₃₀, preferably a C₄ to C₂₀, and even morepreferably, a C₅ to C₁₅, alcohol is practical. The alcohol should begenerally straight chain, meaning little or, preferably, no branching,in order to afford favorable biodegradability. Optimally, each carbonatom of the alcohol should have at least one hydrogen to providesuperior biodegradability. Alfol 8-10 (a mixture of C₈ to C₁₀ alcohols)has been found to be especially suitable.

The alcohol may be ethoxylated by standard techniques. Thus, the alcoholmay be heated with a base or amine catalyst to about 100° to about 150°C., depending on the catalyst, and ethylene oxide added thereto. Theresulting ethoxylated alcohol is of the form R"O--(CH₂ CH₂ O)_(z) H,wherein R" is a substituted or unsubstituted alkyl, aryl or aralkylgroup of from about two to about thirty, preferably from about four toabout twenty, more preferably from about five to about fifteen, carbons.Desirably, R" is an alkyl group, especially an unsubstituted alkyl groupof from about two to about thirty, preferably from about four to abouttwenty, more preferably from about five to about fifteen, carbons. Asnoted, it is optimal that each carbon of R" should have at least onehydrogen bonded thereto. The relative proportion of ethylene oxide toalcohol depends on the degree of ethoxylation desired to providesufficient water-solubility and biodegradability. Generally, the heavierthe alcohol, the greater the degree of ethoxylation required. Althoughany degree of ethoxylation is feasible, economic practicalities suggestthat it is not desirable that more than about ten moles of ethyleneoxide per mole of alcohol be used. Therefore, z is preferably an integerfrom one to about ten, more preferably from about two to about five,especially about two to about three.

A phosphate ester then is prepared from the ethoxylated alcohol.Techniques for preparation of phosphate esters are well known. See, forexample, U.S. Pat. No. 4,722,805 to Martin (a present co-inventor) andU.S. Pat. No. 3,846,071 to Redmore, which are incorporated herein byreference. The ester may be prepared by reacting the ethoxylated alcoholwith polyphosphoric acid at a temperature of from about 75° to about100° C. to form a mono-ester. Alternatively, the phosphate ester may beprepared by a reaction of the ethoxylated alcohol with phosphoricanhydride to form a di-ester/mono-ester mixture.

As with the compositions and method taught in U.S. Pat. No. 3,846,071,the imidazoline may be used without the phosphate ester for corrosioninhibition in sour systems, but it has been found that a blend of theimidazoline and phosphate ester yields improved corrosion inhibition insuch systems. Sweet systems should be treated with the blend. The weightratio of imidazoline to phosphate ester is preferably about 1:1 to about4:1, most preferably about 2:1.

The blend (as well as imidazoline employed without phosphate ester) mayalso incorporate a solvent, particularly an environmentally compatiblesolvent such as water, ethylene glycol or propylene glycol. The blendshave been found generally to be water-soluble, but some compositions,especially those of a low degree of ethoxylation, have been found to bemerely water-dispersible. In such cases, inclusion of solvents such asisopropyl alcohol may result in a clear solution, but environmentalconsiderations render use of isopropyl alcohol undesirable. Typically,the solvent may make up from about a third to about two thirds,preferably about a half, by weight of the additive. Thus, if no othercomponents are present, the weight ratio of actives (i.e., combinationof imidazoline and phosphate ester if a blend is used; imidazoline if nophosphate ester is present) to solvent is from about 2:1 to about 1:2,preferably about 1:1.

Each component of the additive composition should be selected withconsideration for environmental compatibility. Thus, as notedpreviously, in the specification, the imidazoline and the diamine andfatty acid from which it is formed, the phosphate ester and the alcoholfrom which it is formed, and the solvent, should be nontoxic to aquaticlife in the environment in which it is to be employed. As discussed,this consideration has dictated preferred aspects of these variouscomponents and reactants, with particular concern for not onlynon-toxicity, but also water-solubility, biodegradability, and so forth.Of course, it is undesirable to include any further component that wouldbe counter to these concerns. Accordingly, it is preferable that theadditive composition be free of toxic components (components toxic tothe aquatic life in the medium in which it is to be employed) andcomponents that are insoluble in water or are environmentally harmfulsubstances that are not biodegradable.

The additive composition may be employed by simple addition to themedium to be treated. Such media are aqueous and include sea water as,for example, in off-shore oil production. Preferably, the effectiveconcentration of inhibitor actives (that is, the concentration at whichcorrosion inhibition is provided) is in the range of from about 10 toabout 1,000 ppm by weight. Of course, greater concentrations may beemployed, but may be wasteful over-dosage or, at very high dosages,might raise the environmental impact to significant levels. Regardless,rapid dilution takes place quickly in applications in such media asoverboard brine from off-shore oil production (as in the North Sea),leading to even less environmentally significant dosages.

The following examples describe preferred embodiments of the invention.Other embodiments within the scope of the claims herein will be apparentto one skilled in the art from consideration of the specification orpractice of the invention as disclosed herein. It is intended that thespecification, together with the examples, be considered exemplary only,with the scope and spirit of the invention being indicated by the claimswhich follow the examples. In the examples, all percentages are given ona weight basis unless otherwise indicated.

EXAMPLE 1

Corrosion rate tests were conducted on inhibitors of this invention(designated herein as Inhibitor A) versus a blend of a quaternaryammonium compound and an acetate salt of an imidazoline, which is acommercial corrosion inhibitor sold by Petrolite and is identifiedherein as Inhibitor X. The blend of this invention tested was a 1:2:6 byweight blend of (1) a phosphate ester derived from reaction ofpolyphosphoric acid and Alfol 8-10 ethoxylated with 3 moles of ethyleneoxide per mole Alfol 8-10, (2) an imidazoline that corresponds to theformula ##STR2## wherein R' is the residue from oleic or linoleic acid(the imidazoline was derived from a mix of oleic and linoleic acid), and(3) propylene glycol plus water.

The toxicity (EC₅₀) of Inhibitor X was measured as 0.18 ppm versus 1 to10 for Inhibitor A. The biodegradability (BOD-28) of Inhibitor X wasfound to be 28% versus 100% for Inhibitor A. In fact, for Inhibitor A100% degradation was found by thirteen days. Partitioning of Inhibitor Xwas determined to be about 2 versus about 0 for Inhibitor A. The solventevaporation factor (YL) of Inhibitor X was measured at 2 versus 0 forInhibitor A. The flash point of Inhibitor X was found to be 27° C. andover 93° C. for Inhibitor A.

Kettle tests and loop tests were conducted. For the kettle tests,various amounts of inhibitors were added to aqueous solutions of 3%sodium chloride, which were then stirred mildly for 24 hours at roomtemperature. "Sweet" test solutions were sparged continuously withcarbon dioxide. "Sour" test solutions were sparged with carbon dioxideand then enough Na₂ S.H₂ O was added to give a hydrogen sulfideconcentration of ₅₀ ppm and a pH of 5.5. The sour solutions were thensealed. AISI-type 1020 steel corrosion coupons (9 cm²) were weighed,added to the solutions before the stirring, removed from the solutionsat the completion of the stirring, cleaned and reweighed. Corrosionrates were calculated based on the weight loss. The results were asfollows, with the inhibitor concentration (dose) being given in ppm, thecorrosion rates being given in mils per year (mpy) and many of theresults shown being averages of duplicate runs.

    ______________________________________                                        Inhibitor Dose         Sweet    Sour                                          ______________________________________                                        None      --           40       45                                            Inhibitor X                                                                             250          6.3      2.4                                           Inhibitor X                                                                             50           12       8.2                                           Inhibitor A                                                                             250          6.1      1.8                                           Inhibitor A                                                                             50           9        2.9                                           Inhibitor A                                                                             25           14       4.8                                           ______________________________________                                    

For the loop tests, carbon dioxide saturated mixed brine was pumpedthrough cylindrical coupons for 24 hours in a pump-around loop. Pressurewas maintained at 45 psig and the temperature was maintained at 140° F.Flow was maintained at a sufficient rate to give a high shear rate;namely, 47, 79 or 175 Pa (pascals), depending on the trial. Thefollowing results were obtained, with the inhibitor concentration (dose)being given in ppm and the corrosion rates being given in mils per year(mpy).

    ______________________________________                                        Inhibitor  Dose    47 Pa      79 Pa  175 Pa                                   ______________________________________                                        None       --      1,000      1,100  2,100                                    Inhibitor X                                                                              250     16         21     14                                       Inhibitor X                                                                              25      49         58     69                                       Inhibitor A                                                                              50      18         22     27                                       Inhibitor A                                                                              25      19         31     24                                       ______________________________________                                    

EXAMPLE 2

Further corrosion inhibitors were prepared in accordance with thisinvention. The inhibitors correspond to the combinations identified inthe following table, "# of EtO" refers to the number of ethoxy units inthe n-polyethoxy chain on the imidazoline (all imidazolines were derivedfrom a mix of oleic and linoleic acids), "Phos." refers to whether thephosphate ester was derived from polyphosphoric acid ("Poly") orphosphoric anhydride ("P₂ O₅ "), "Prop" refers to the weight ratio ofimidazoline to phosphate ester to solvent, "IPA" refers to isopropylalcohol, "NBA" refers to n-butyl alcohol, "EG" refers to ethylene glycoland "PG" refers to propylene glycol:

    ______________________________________                                        Inhibitor                                                                             Imidazoline                                                                             Phosphate Ester                                             Prop.   # of EtO  Alcohol   Phos.  Solvent                                    ______________________________________                                        B       3         No phosphate ester                                                                           None                                         1:0:0                                                                         C       6         No phosphate ester                                                                           None                                         1:0:0                                                                         D       9         No phosphate ester                                                                           None                                         1:0:0                                                                         E       12        No phosphate ester                                                                           None                                         1:0:0                                                                         F       9         Alfol 8-10                                                                              Poly   Water                                      1:1:2                                                                         G       9         Tri-decyl P.sub.2 O.sub.5                                                                      Water + IPA                                1:1:2:1.5                                                                     H       9         Alfol 8-10                                                                              Poly   Water                                      2:1:3                                                                         I       6         Alfol 8-10                                                                              Poly   Water                                      4:1:5                                                                         J       6         Tri-decyl P.sub.2 O.sub.5                                                                      Water + IPA                                2:1:3:1                                                                       K       6         Tri-decyl P.sub.2 O.sub.5                                                                      Water + IPA                                2:4:6:5                                                                       L       3         Tri-decyl P.sub.2 O.sub.5                                                                      NBA                                        2:1:3                                                                         M       9         Alfol 8-10                                                                              Poly   Water + EG                                 8:4:15:9                                                                      N       9         Alfol 8-10                                                                              Poly   Water + PG                                 8:4:15:9                                                                      ______________________________________                                    

These inhibitors were then compared with a variety of other additives incorrosion tests in sweet and sour systems. The following results wereobtained, with the inhibitor concentration (dose) being given in ppm andthe corrosion rates being given in mils per year (mpy).

    ______________________________________                                                               Corrosion Rate                                         Inhibitor            Dose    Sweet   Sour                                     ______________________________________                                        None                 --      40      45                                       Oleyl sarcosine*     250     23      25                                       Sodium lauryl sarcosine (30%                                                                       700     17      14                                       solution)                                                                     Sodium lauryl sarcosine + Betaine                                                                  250     13      5.7                                      Coco-benzo-quat. + pyridine +                                                                      250     18      2.3                                      phosphate ester                                                               Coco-benzo-quat. + pyridine +                                                                      60      11      3.2                                      phosphate ester                                                               OEX-6673-4 from Westvaco                                                                           250     8.4     4.6                                      Unethoxylated imidazoline*                                                                         250     14      12                                       Inhibitor B*         250     9       3.1                                      Inhibitor C          250     11      5                                        Inhibitor D          250     11      5                                        Inhibitor E          250     9.6     5                                        Inhibitor E          100     7       3.1                                      Inhibitor F          250     7.4     2.4                                      Inhibitor G*         350     5.5     3.7                                      Inhibitor G*          70     9.4     4.8                                      Inhibitor G           35     9       11                                       Inhibitor H          250     6.1     1.8                                      Inhibitor I          250     7       5.5                                      Inhibitor I           50     8.3     6.6                                      Inhibitor J          300     5.9     7                                        Inhibitor J          1,000   6.3     NA                                       Inhibitor K*         400     6.6     4.4                                      Inhibitor L*         250     5.8     2.6                                      Inhibitor M          250     7.4     2.6                                      Inhibitor N          250     6.1     1.8                                      ______________________________________                                         *Inhibitor was dispersible as opposed to soluble at the noted dose.      

EXAMPLE 3

Shake tests were conducted on Inhibitors L and M of Example 3 as well asan acetate salt of imidazoline in various media. Medium 1 was formed bymixing kerosene (20 ml) into an aqueous solution (100 ml) of 5% NaCl.Medium 2 was an aqueous solution of 6% NaCl and 2% CaCl₂.H₂ O. Medium 3was an aqueous solution of 6% NaCl and 4% CaCl₂.H₂ O. The followingresults were obtained, wherein Dose is the concentration of theinhibitor added and the Appearance is the appearance of the top of themedium (Top), the kerosene phase (Kerosene), the interface between thekerosene phase and the water phase (Interface) and the water phase(Water) after five minutes.

    __________________________________________________________________________               Appearance                                                         Medium                                                                             Inhibitor                                                                           Dose Top Kerosene                                                                             Interface                                                                          Water                                         __________________________________________________________________________    1    None  --   Clear                                                                             Clear  Sharp                                                                              Slight Haze                                   1    Inhibitor X                                                                         1%   Foam                                                                              Haze   None Haze                                          1    Inhibitor M                                                                         1%   Clear                                                                             Half Clear                                                                           Sharp                                                                              Haze                                          1    Inhibitor L                                                                         1%   Clear                                                                             Almost Clear                                                                         Sharp                                                                              Haze                                          2    Inhibitor M                                                                         500 ppm                                                                            Fair            Clear                                         2    Inhibitor M                                                                         1%   Fair            Clear                                         3    Inhibitor M                                                                         1%   Fair            Slight Haze                                   __________________________________________________________________________

In view of the above, it will be seen that the several advantages of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above methods and compositionswithout departing from the scope of the invention, it is intended thatall matter contained in the above description shall be interpreted asillustrative and not in a limiting sense.

What is claimed is:
 1. A method for inhibiting corrosion in an aqueousmedium containing Skeletonema costatum, fish, algae or a combinationthereof, the method comprising incorporating into the medium an amountof a water-soluble corrosion inhibitor sufficient to inhibit corrosion,the corrosion inhibitor comprising an N-ethoxy,2-substitutedimidazoline, the N-ethoxy substituent having from one to about thirtyethoxy units and the 2-substituent being an unsaturated orpolyunsaturated fatty chain of from about six to about thirty carbons,and a phosphate ester derived from a water-soluble oxyethylated,straight chain alcohol of from about two to about thirty carbons.
 2. Amethod as set forth in claim 1 wherein the corrosion inhibitor furthercomprises a solvent selected from the group consisting of water,ethylene glycol and propylene glycol.
 3. A method as set forth in claim1 wherein the fatty chain is straight and has from about twelve to abouttwenty carbons.
 4. A method as set forth in claim 3 wherein the fattychain is the residue from a fatty acid selected from the groupconsisting of oleic acid, linoleic acid and eladeic acid.
 5. A method asset forth in claim 3 wherein the N-ethoxy substituent has from about twoto about eighteen ethoxy units.
 6. A method as set forth in claim 1wherein the medium is sweet.
 7. A method as set forth in claim 1 whereinthe corrosion inhibitor has an EC₅₀ for Skeletonema costatum greaterthan 1 ppm by weight.
 8. A water-soluble, biodegradable corrosioninhibitor composition comprising (1) an N-ethoxy,2-substitutedimidazoline, the N-ethoxy substituent having from one to about thirtyethoxy units and the 2-substituent being an unsaturated orpolyunsaturated fatty chain of from about six to about thirty carbons,(2) a phosphate ester derived from a water-soluble oxyethylated,straight chain alcohol of from about two to about thirty carbons, and(3) a solvent selected from the group consisting of water, ethyleneglycol and propylene glycol.
 9. A corrosion inhibitor as set forth inclaim 8 wherein the corrosion inhibitor has an EC₅₀ for Skeletonemacostatum greater than 1 ppm by weight.
 10. A method for inhibitingcorrosion in a non-sweet aqueous medium containing Skeletonema costatum,fish, algae or a combination thereof, the method comprisingincorporating into the medium an amount of a water-soluble corrosioninhibitor sufficient to inhibit corrosion, the corrosion inhibitorcomprising an N-ethoxy,2-substituted imidazoline, the N-ethoxysubstituent having from one to about thirty ethoxy units and the2-substituent being an unsaturated or polyunsaturated fatty chain offrom about six to about thirty carbons.
 11. A method as set forth inclaim 10 wherein the medium is sour and the corrosion inhibitor furthercomprises a phosphate ester derived from a water-soluble oxyethylated,straight chain alcohol of from about two to about thirty carbons.
 12. Amethod as set forth in claim 11 wherein the imidazoline and thephosphate ester are present in the corrosion inhibitor in a weight ratioof from about 1:1 to about 4:1.
 13. A method as set forth in claim 11wherein the corrosion inhibitor further comprises a solvent selectedfrom the group consisting of water, ethylene glycol and propyleneglycol.
 14. A method as set forth in claim 13 wherein the fatty chain isstraight and has from about twelve to about twenty carbons.
 15. A methodas set forth in claim 14 wherein the fatty chain is the residue from afatty acid selected from the group consisting of oleic acid, linoleicacid and eladeic acid.
 16. A method as set forth in claim 14 wherein theN-ethoxy substituent has from about two to about eighteen ethoxy units.17. A method as set forth in claim 10 wherein the corrosion inhibitorhas an EC₅₀ for Skeletonema costatum greater than 1 ppm by weight.